1. Field of the Invention
This invention relates, in general, to novel and improved nickel base single crystal alloys and, in particular, to such alloys having the combined properties of improved oxidation/corrosion resistance and high strength at elevated temperatures. More specifically, the present invention relates to novel and improved nickel base single crystal alloys which retain their high temperature mechanical properties after prolonged or repeated exposure to elevated temperatures, the single crystal alloys being capable of being cast into desired shapes, such as turbine blades, vanes and other parts used in high temperature gas turbine engines, and to improved nickel base single crystal alloys which exhibit improved hot corrosion resistance. Even more specifically, the present invention relates to the above type of improved and novel nickel base single crystal alloys which can be heat treated to improve stress-rupture life of such alloys coated with conventional coatings with an accompanying heat treatment to impart high temperature oxidation/sulfidation resistance thereto without the formation of deleterious phases at the alloy/coating substrate interface.
2. Description of the Prior Art
Nickel base superalloys which have been commonly used over the years to fabricate gas turbine engine components typically contain, aside from certain levels of chromium, cobalt, aluminum, titanium and refractory metals (e.g., tungsten, molybdenum, tantalum and columbium) other elements which act as grain boundary strengtheners, such as carbon, boron and zirconium. These types of gas turbine blades are most commonly formed by casting, and the process most often utilized produces parts having equiaxed non-oriented grains. Since the high temperature properties of metals are generally dependent upon grain boundary properties, efforts have been made to strengthen the boundaries of these alloys by addition of carbon, boron and/or zirconium, as discussed above, or to reduce or eliminate the grain boundaries transverse to the major stress axis of the part. One method of eliminating such transverse boundaries is directional solidification as described in U.S. Pat. No. 3,260,505. The effect of directional solidification is to produce an oriented microstructure of columnar grains whose major axis is parallel to the stress axis of the part and which has minimal or no grain boundaries perpendicular to the stress axis of the part.
A further extension of this concept is the utilization of single crystal parts in gas turbine blades as, for example, described in U.S. Pat. No. 3,494,709. The obvious advantage of the single crystal blade is the complete absence of grain boundaries and therefore the absence of these potential weak areas of a metallic structure. Thus, the mechanical properties of the single crystal are completely dependent upon the inherent mechanical properties of the material. While single crystal nickel base alloys are generally known, there exists a need for such alloys having a varied combination of properties including improved mechanical strength, especially over prolonged and/or repeated exposure to elevated temperatures, improved oxidation/corrosion resistance (especially at elevated temperatures) and the ability to be cast to desired shapes, such as turbine blades and parts.
There exists in the patent literature many examples of nickel base superalloys and nickel base single crystal superalloys, methods for their fabrication and methods for their heat treatment. Examples of some of these prior art patents and the basic technology that they describe, is as follows:
In commonly assigned and copending U.S. Ser. No. 678,802, filed Dec. 6, 1984 for "High Strength Nickel Base Single Crystal Alloy" there, now U.S. Pat. No. 4,677,035 is described nickel base single crystal alloys which are improved, with regard to their strength and corrosion properties at elevated temperatures, by the addition of certain elements in certain amounts. Schweizer et al., U.S. Pat. No, 4,222,794, discloses a nickel base single crystal superalloy for use at elevated temperatures having a restricted composition consisting of 4.5-6.0% chromium, 5.0-5.8% aluminum, 0.8-1.5% titanium, 1.7-2.3% molybdenum, 4.0-6.0% tungsten, 5.5-8.0% tantalum, 1.0-5.0% rhenium, 0.2-0.6% vanadium, 0-7.0% cobalt and the balance nickel. This patent also discloses a method of heat treating the alloys described therein at a specific temperature range. Although the Schweizer et al patent discloses a single crystal alloy, said alloy differs chemically from the alloy of the present invention. For example, the alloy of the present invention is significantly higher in chromium content, titanium content and titanium to aluminum ratio, and does not contain rhenium and vanadium.
Gell et al., U.S. Pat. No. 4,116,723, discloses single crystal nickel base super alloys free from intentional additions of cobalt, boron, and zirconium. Gell at al. discusses the avoidance of the development in the single crystal alloys of deleterious phases after long term exposure at elevated temperatures (i.e., alloy instability), the phases being of two general types, sigma and mu. Sigma is undesirable because of its brittle nature while mu is undesirable because the phase ties up large amounts of the refractory solid solution strengtheners thus weakening the remaining alloy phases. The sigma and mu phases are termed TCP phases for topologically closed packed phases and one of their common properties is that they all contain cobalt. Gell et al. eliminates cobalt in the claimed single crystal nickel base alloys to inhibit the formation of TCP phases therein. Unexpectedly, the presence of cobalt in the single nickel base alloys of the present invention does not induce the formation of TCP phases. Also, the ratio of titanium to aluminum disclosed by Gell et al. is lower than that in the alloy of the present invention. While U.S. Pat. No. 4,116,723 relates to heat treatment of single crystal alloys, precipitation-hardened alloys having the high temperature mechanical properties of the instant invention (e.g., retention of high temperature properties after prolonged or repeated exposure to elevated temperature) are not obtained.
Shaw, U.S. Pat. No. 4,207,098 discloses a relatively low-strength nickel base polycrystalline alloy consisting essentially of 14-22% chromium, 5-25% cobalt, 1-5% tungsten, 0.5-3% tantalum, 2-5% titanium, 1-4.5% aluminum (with the sum of titanium plus aluminum being 4.5-9%), 0-2% niobium, 0.31-1.2% boron, 0-3.55% molybdenum, 0-0.5% zirconium, 0-0.2% in total yttrium or lanthanum or both, 0-0.1% carbon, and the balance nickel. The Shaw polycrystalline alloy, which must contain boron, is chemically different from the single crystal alloy of the present invention.
Gosh, U.S. Pat. No. 4,126,495, discloses a low strength nickel base polycrystalline alloy consisting essentially of 6.75-10.0% aluminum, 8.0-12.0% chromium, 0.8-2.5% titanium, 2.0-6.0% cobalt, 2.5-4.0% molybdenum, 0.95-4.85% tantalum, 0-1.25% tungsten, 0-0.6% columbium, 0-1.0% carbon, 0-1.0% boron, 0-0.8% zirconium, 0-1.0% rare earths, 0-1.0% beryllium and the balance nickel. The Gosh polycrystalline alloy contains lower amounts of tungsten and higher amounts of molybdenum than the single crystal alloy of the present invention.
Thielemann, U.S. Pat. No. 2,948,606, discloses a low-strength nickel-chromium-cobalt base polycrystalline alloy composed of about 15.0-25.0% chromium, 5.0-30.0% cobalt, 0.5-4.0% titanium, 2.0-5.0% aluminum, 1.0-5.0% columbium or tantalum or mixtures thereof, 5.0-11.0% tungsten and the balance essentially nickel. The Thielemann polycrystalline alloy which contains significantly higher amounts of chromium, a lower combined titanium-aluminum content and no molybdenum is chemically different from the single crystal alloy of the present invention.
Dalai et al., U.S. Pat. No. 3,807,993, discloses a polycrystalline material with a significantly higher cobalt content than the single crystal alloy of the present invention and, further, containing grain boundary strengtheners such as carbon, boron, zirconium and hafnium.
Two Restall et al. patents, U.S. Pat. Nos. 3,902,900 and 3,922,168 disclose an intermetallic compound material containing a first group including nickel and at least one of the elements chromium, cobalt, molybdenum and tungsten within the range of 72-83 atomic percent, and a second group containing aluminum (12-26 atomic percent) in combination with at least one of the elements titanium, niobium, and tantalum within the range of 17-28 atomic percent. U.S. Pat. Nos. 4,249,943; 4,043,841; 3,785,809; 3,615,376; and 3,486,887 disclose alloys containing nickel, cobalt, chromium and aluminum together with one or more of the following elements: manganese, silicon, carbon, niobium, boron, zirconium, among others. U.S. Pat. Nos. 2,971,838; 3,276,866; 3,926,586; 3,973,952; and 4,268,308 disclose a variety of compositions containing nickel, chromium and aluminum with one or more of the following elements: zirconium, carbon, columbium, boron and silicon, among others.
U.S. Pat. Nos. 3,257,178, Re. 29,547; 4,108,647; 4,219,592; 4,245,698; 4,288,247; 4,339,509; 4,346,137; 4,400,209; 4,400,210; 4,400,211, 4,400,349; 4,421,571 and 4,615,864 disclose alloys containing nickel, chromium and cobalt together with one or more of the following elements: molybdenum, tungsten, titanium, hafnium, yttrium, lanthanum, manganese and silicon, amongst others.
U.S. Pat. Nos. 4,219,592 and 3,257,178 disclose ranges of element additions to nickel base alloys which not only differ chemically from the alloys of the present invention, but also they fail to disclose any recognition of or even suggestion of the critical ranges of this invention.
U.S. Pat. Nos. 4,198,442 and 4,227,925 disclose alloys containing nickel together with yttrium, hafnium, and rare earth elements.
U.S. Pat. No. 4,374,084 discloses corrosion resistant alloy compositions containing manganese and silicon and U.S. Pat. No. 4,569,824 discloses a nickel base superalloy containing manganese.
Still other patents in the nickel base superalloy area include U.S. Pat. Nos. 2,621,122; 2,781,264; 2,912,322; 2,994,605; 3,046,108; 3,116,412; 3,188,204; 3,287,110; 3,304,176; and 3,322,534.